Suggestions for improving the project of producing biogas from straw fermentation
Thread Content
In recent years, significant progress has been made in researching the use of straw as a fermentation substrate for biogas production, and some practical techniques and experience have been developed. The best results are achieved by crushing the straw (rather than chopping it) and undergoing anaerobic fermentation after pretreatment with decomposing bacteria. It not only has a high gas production rate but also solves the problem of difficult discharge. In practice, they proposed four suggestions for improvement: First, appropriate improvements to the pool design. Based on the **standard pressure-water digester, the pressure chamber is made smaller and deeper. Second, install a mechanical stirrer. Stir upward and downward to promote complete and thorough fermentation and degradation of the materials in the tank. Third, improve the strain. Make the fiber-degrading bacteria more storage-resistant and improve the packaging to facilitate use by individual households each time. Fourth, **more financial support should be provided. As a project for the comprehensive utilization of straw in regions where grain is produced in large quantities, the production of biogas from straw faces high costs associated with building the necessary tanks. It is recommended that **for each farmer who builds a biogas tank using straw as raw material, in addition to the subsidies provided under the national biogas program, an additional subsidy of 200–300 yuan be granted.
Straw is widely available in rural areas and can be used in large quantities to generate gas in biogas digesters. However, using straw for fermentation to produce gas presents two major challenges: one is slow decomposition, resulting in a low daily gas output ； Second, it tends to form a crust, affecting the normal operation of the biogas digester. It is precisely because of this obstacle that straw has long struggled to be utilized in biogas production. With the development of society and advancements in science and technology, more attention has been paid to the challenge of tapping into the potential of straw, and many effective ways of utilizing straw have been found, such as centralized gasification of straw and the production of biodiesel from it. Among these, the method proposed by Beijing Hebaiyi Co., Ltd., which involves using composite bacteria to pre-treat straw before using it for fermentation in biogas digesters, is the simplest, most reasonable, and most practical approach. The pre-treatment methods for straw composite bacteria are introduced as follows. Materials preparation: 2,000 kilograms of dry, mold-free straw that has been crushed; 3,000 kilograms of straw-specific microorganisms; 15 kilograms of ammonium carbonate; and 1,000 kilograms of inoculum. Soaking: Sprinkle drinking water on the straw and stir it until its moisture content reaches 30% – at this point, it will form a clump when squeezed in the hand but fall apart when placed on the ground. Then, pile it up and cover it for later use. Seed treatment: After soaking for 1 day, evenly sprinkle in the microbial strain and ammonium carbonate. If the straw is too dry, it should be moistened appropriately; then it should be piled up to a height of 1 meter, holes should be made for ventilation, and it should be covered with white plastic sheeting for fermentation. During this period, the temperature should be monitored regularly; it is advisable to keep the fermentation temperature around 60 degrees Celsius. After 3 days of mixing the seeds in the tank, choose a sunny and warm day to quickly add the straw and the inoculant to the biogas digester; the inoculant should be spread evenly among the straw. Finally, drill holes to seal the biogas digester. Gas production: After 1 day of fermentation in the tank, add water to a pressure level close to zero; after another 2 days, gas production can begin normally for use. Straw treated with microbial agents becomes softer in texture and has an increased density. At the same time, due to the action of the composite bacteria, the wax layer on the straw is removed, resulting in a significant increase in both the gas production rate and the amount of gas produced. At the same time, due to the action of the composite bacteria, the density of the straw increases; as a result, it does not float on the surface of the liquid and form a crust after being added to the tank, thereby effectively solving the two major challenges associated with the use of straw.
There is now a rapid-decomposition agent for straw; is it feasible to use straw to produce biogas?
The combustible gas produced in straw gasification furnaces is what we call straw gas; it contains impurities and has a too high temperature, making it unsuitable for direct use by users. The impurities in straw gas are mainly ash, fine carbon particles, tar, and moisture. These impurities have a significant impact on the use of straw gas; tar in particular is harmful for the following reasons: (1) Tar accounts for about 5% of the total energy in straw gas, and when the straw gas is cooled, it becomes difficult for tar to be burned along with the gas. (2) The tar in straw gas condenses at low temperatures, and it easily combines with impurities such as water, carbon particles, and ash, thereby blocking gas transmission pipelines, jamming valves and pump rotors, and corroding metals. (3) Tar is difficult to burn completely, resulting in particles such as carbon black, which cause significant damage to straw gas utilization equipment such as internal combustion engines and gas turbines. (4) Tar and the odors produced upon its combustion are harmful to the human body. Methods for purifying straw gas Since the impurities in straw gas are diverse and complex, it is not possible to use a single method for purifying straw gas. Usually, several purification methods are used in combination. The main purpose of purifying straw gas is to remove ash, carbon particles, moisture, tar, and to cool it down, and the technologies employed are also aimed at addressing these aspects. 1. Dust removal The dust removal in straw gas involves eliminating the ash and fine carbon particles remaining in it. There are generally two methods used for this purpose: dry dust removal and wet dust removal. (1) Dry dust removal. The advantage of dry dust removal is that the dust particles separated from the straw gas remain at their original temperature and stay dry, without mixing with moisture. Dry dust removal is further divided into mechanical force dust removal and filtration dust removal. Mechanical dust removal utilizes the inertial effect to separate particles from the airflow; the minimum particle size that can be removed in this way is 5 microns. The most common one is the cyclone dust collector. Filter dust removal uses porous materials to remove dispersed solid particles from gases. Filter dust removal can effectively capture particles ranging from 1 to 0.1 microns, making it the most efficient and stable method among various separation techniques. It’s just that the filtration speed cannot be high, the equipment is relatively large, and it is difficult to discharge the material and clean the dust. Filters are generally used for final separation. (2) Wet dust removal. Wet dust removal uses a liquid (usually water) as a capture medium to trap impurities in gases; as the gas flow passes through a liquid layer, film, or droplets, the particles contained in it adhere to the liquid and are thus separated. Commonly used devices include bubble towers, spray towers, packed towers, Venturi scrubbers, etc.
The most widely used and technically mature method at present is the gasification of straw using air as the gasifying agent. The main characteristic of this straw gas is its high nitrogen content, which results in a lower calorific value. The main combustible components in straw gas are carbon monoxide and hydrogen, along with a small amount of methane. Methane and other hydrocarbon compounds in ordinary gas account for the vast majority, which is why it has a high calorific value. Another feature that distinguishes straw gas from ordinary gas is its purification effect. The production of ordinary coal gas involves large-scale operations, with relatively sophisticated purification systems, resulting in highly clean gas after treatment. The straw gasification projects currently under construction in our country are generally not very large in scale, and their purification systems are relatively simple. Whether used for power generation or cooking, their purification efficiency is inferior to that of ordinary gas, which has a significant impact when these systems are put into use. Another characteristic of straw gas is that it has an odor. The odor of ordinary gas and natural gas is intentionally added when it is supplied to residents for cooking purposes, to ensure safety ； The odor of straw gas is due to the smell of small amounts of tar gases remaining in it; therefore, even without adding any odorants to the straw gas, it can still be detected when it leaks.